Data rates achievable in a mobile telecommunications network have been increased using base stations with two or more transmit antennas that provide one or multiple spatial streams to one or more mobile devices. The base station applies precoding weights to a signal to be transmitted on the different transmit antennas. For example, Long Term Evolution (LTE) introduced a further Transmission Mode, TM9, in Release 10, which supports up to 8 transmit antennas for a downlink channel with up to 8 spatial streams (which are also referred to as layers) from the base station to a User Equipment (UE).
The base station relies upon Channel State Information (CSI) reported by the UE for specifying a precoder, i.e., specifying the precoding weights for each transmit antenna and each layer. Codebook-based precoding uses a common codebook comprising a set of precoders. The codebook is known at both the transmitting base station and the receiving UE, so that one precoder is specified in the CSI report by reference to the codebook.
As the number of transmit antennas is increased, the size of the codebook (i.e., the number of codebook entries) increases. Furthermore, the size of each codebook entry (i.e., the number of precoding weights) increases with the number of layers. Signal processing for determining the precoder to be reported in the CSI requires memory resources and computational resources at the UE. The signal processing resources for a conventional implementation approximately scale with the size of the codebook and the size of the entries. For example in an LTE implementation supporting 8 transmit antennas and 2 layers, the codebook according to Table 7.2.4-2 in standard document 3GPP TS 36.213 (Version 11.4.0) includes 256 precoders, each precoder being represented by a matrix of dimension 8×2. In other words, 256 sets of precoding weights for the different transmit antennas have to be processed for each of the 2 layers. In the case of 4 transmit antennas, 16 sets of precoding weights for the different transmit antennas have to be processed for each of the 2 layers according to Table 6.3.4.2.3-2 in standard document 3GPP TS 36.211 (Version 12.0.0). The increase in codebook size by a factor of 16 for doubling the number of transmit antennas indicates a strong non-linear increase in resource consumption caused by increasing the number of transmit antennas.